HK1178841A - Method for the application of a conformal nanocoating by means of a low pressure plasma process - Google Patents

Description

Method for applying conformal nanocoating by low pressure plasma process

Technical Field

The present invention relates to a low pressure plasma process for conformally applying a nanocoating over a three-dimensional structure. The invention also relates to the application of such conformal coatings on three-dimensional nanostructures made of different materials, in particular on three-dimensional structures comprising conductive and non-conductive elements.

Background

Most electronic devices are predominantly three-dimensional structures of conductive and electrically insulating materials. Such electronic devices include not only devices, but also assemblies, bare and assembled Printed Circuit Boards (PCBs), and individual components such as integrated circuits and transistors. The conductive portions of such structures are typically composed of a metal such as copper, aluminum, silver, or gold, or a conductive polymer or semiconductor material. The non-conductive portions or insulators of these structures are typically composed of a polymer or paper-based material such as polyimide, polytetrafluoroethylene, silicone or polyamide with or without glass fiber reinforcement. The insulator in the structure or assembly may also comprise a ceramic material such as glass. During the life cycle of electronic devices, they are subject to various forms of contamination. The electrical conductivity of certain materials may be reduced by atmospheric corrosion and contamination may cause conductive pathways to be established between adjacent traces or conductors, dendrites being an example of such a mechanism.

Electronic devices are increasingly used in harsh and contaminated environments, with conformal coatings added to protect them from contamination. Such conformal coatings are typically non-conductive.

Conventional conformal coatings have been used for assembled circuit boards and assembled units, but they can also be used on bare circuit boards to prevent oxidation of the copper pads prior to soldering and to provide a degree of protection from contamination after the assembly process.

The minimum requirements for a conformal coating are that it should provide an effective barrier between the device and the environment, and that it should be electrically insulating. The conformal coating should prevent physical contamination, which could result in conductive growth across non-conductive portions of the structure or device, for example, which could result in a short circuit. Examples of such contamination are dendrites that grow across the surface under certain conditions, and "tin whiskers" (tin whiskers) that can grow through the air between component wires. The coating must also ensure that the metal is not oxidized in air or corroded in other ambient gases. The coating should prevent such problems during the life cycle of the electronic device. As the environment becomes more challenging, the requirements for conformal coatings become more stringent. The coating must withstand high humidity, high temperature and high contamination including dust, salts, acids, solvents, etc.

Conventional conformal coatings are polymers based on silicone (e.g. JP 60047024), epoxy (e.g. EP 0187595), acrylic (e.g. EP 0492828) or polyurethane (e.g. CA 1144293) and are typically tens to hundreds of μm thick. They are typically applied by spraying or dipping the device. Before the coating is applied, it is essential that the device is first dried and thoroughly cleaned. After the coating is applied, there is usually an additional drying process. Therefore, the production process has several different steps, requires a large amount of energy and chemicals, and is therefore very environmentally damaging. The application of conventional coatings to complex three-dimensional structures is not easy and may even be impossible, especially as the scale of these structures becomes increasingly miniaturized. Many conventional coatings are fragile, which makes them unsuitable for flexible structures. With many conventional coatings, a further problem occurs when the device is subjected to repeated thermal cycling, and the coating may become separated from the device due to limited adhesion and differences in expansion characteristics. Many conventional coatings cannot be welded through them and must be removed before repairs or modifications can be made.

Parylene coatings have been developed to provide a local solution to these limitations (e.g., US 6389690). These coatings are applied under vacuum and are therefore more suitable for application to complex three-dimensional structures. The production process is complex, since a solid precursor is used which must be sublimed in order to start with, and then pyrolysis must be carried out before the formation of the useful monomer in the gas phase. Parylene coatings are thinner than conventional conformal coatings, typically less than 1 to tens of microns. Different pretreatments are necessary for proper adhesion of the coating to all parts of the three-dimensional structure including assemblies and subassemblies and to ensure that this adhesion is maintained over the life of the product. Like many conventional conformal coatings, the parylene coating must be removed before repairs can be performed. Such parylene coatings are not easily removable.

Disclosure of Invention

The invention adopts plasma polymerization, which is a process that: a thin polymer film is deposited on any surface that is in contact with the organic monomer plasma that has been generated in the chamber. Depending on the deposition conditions, also referred to as plasma parameters, e.g., power, pressure, temperature, flow rate, etc., the film properties may be adapted to the requirements of the device application.

In the present invention, the nano conformal coating is applied by a low pressure plasma process. Typical layer thicknesses are between 5 and 500nm, preferably between 25 and 250nm, and are therefore much thinner than any of the prior art conformal coating techniques. The coating is thus very suitable for complex and small structures, providing a uniform coating even on the smallest corners.

The plasma polymerization process occurs in a vacuum plasma chamber, wherein the parameters controlling the process include power, pressure, temperature, type of monomer, flow rate, frequency of the plasma generator, and process time. The frequency of the generator for the plasma may be in the kHz, MHz and GHz range, and it may be pulsed or continuous. The number and arrangement of the electrodes may also be varied.

The pressure at which the plasma polymerisation process is carried out is typically between 10 and 1000 mTorr. The process is performed until the desired coating thickness is achieved.

The power employed depends largely on the monomer employed, but can typically vary between 5 and 5000W, and can be applied continuously or pulsed. In the pulsed power mode, the repetition frequency of the pulses is typically between 1Hz and 100kHz, and the duty cycle (mark space ratio) is typically between 0.05 and 50%.

The method of applying power depends primarily on the monomers used. If the molecule is larger and/or less stable, it is easily decomposed by high power, but this results in a poor coating. In this case, good quality coatings can be achieved very well with low power operation and/or by applying pulsed power with a frequency of 10 to 100kHz and a duty cycle between 0.05% and 1%.

Polymerizable particles from the plasma-forming gas are deposited on the surface to form the coating. The monomers used as starting materials are introduced into the plasma in gaseous form, which has been initiated by a glow discharge (glowsdischarge). The excited electrons generated in the glow discharge ionize the monomer molecules. The monomer molecules dissociate to produce free electrons, ions, excited molecules, and radicals. The radicals are absorbed, concentrated and polymerized on the substrate. Electronic and ionic cross-linking or chemical bonding (chemical bonding) and material has been deposited on the surface of the substrate.

The generation of free radicals is preferably achieved by using the monomer gases used in the plasma polymerisation process.

The precursors employed in the present invention are preferably gases and can therefore be readily introduced into the plasma chamber. Alternatively, liquid or solid precursors can be employed at atmospheric or reduced pressure and vaporized by simple heating, typically at temperatures not exceeding 200 ℃. Which by itself appears to be very simple compared to parylene coating processes.

For conformal nanocoating on the above-described electronic devices, a different range of precursors may be employed.

These precursors should preferably contain halogen and/or phosphorus and/or nitrogen and/or silicone, for example

From a precursor CF₄、C₂F₆、C₃F₆、C₃F₈、C₄F₈、C₃F₆C₅F₁₂、C₆F₁₄And/or other saturated or unsaturated hydrofluorocarbons (C)_xF_y) The monomer obtained is used as a monomer for the polymerization of,

from acrylates (e.g. C)₁₃H₁₇O₇F₂) Methacrylic acid salt (e.g., C)₁₄H₉F₁₇O₂) Or a mixture thereof,

monomers obtained from one or more precursors of trimethyl phosphate, triethyl phosphate, tripropyl phosphate or other phosphoric acid derivatives,

monomers obtained from one or more of the precursors ethylamine, triethylamine, allylamine or acrylonitrile, or

-monomers obtained from siloxanes, silanes or mixtures thereof.

The plasma polymerisation process is in fact preferably carried out with the same electrode arrangement and possibly within the same process parameters by means of one or more plasma processes.

In order to obtain good adhesion between the conformal coating and all parts and materials within the structure or assembly, and to maintain adhesion throughout the life of the finished product, it is necessary that all constituent parts and materials of the structure or assembly be cleaned and/or etched if necessary. Cleaning means removing organic contamination on the surface. Etching means that the material itself is removed and/or roughened. Etching may be required to promote good adhesion on certain materials.

Low pressure plasma processes are particularly suitable for this because the reactive gas is able to permeate through the entire three-dimensional structure, unlike liquid-based conformal coatings that are limited by surface tension. The process is also dry and provides a safer environment for the operator. Low pressure plasma processes are generally more environmentally friendly than traditional conformal coating methods.

Depending on the gas or gas mixture selected, cleaning and/or etching may be performed on all constituent materials, including conductors, semiconductors, and insulators. A typical gas used for plasma cleaning or etching is O₂、N₂、H₂、CF₄Ar, He or mixtures thereof.

Significant cost savings can be realized over current conformal coating methods because cleaning, etching, and coating can all occur in the same chamber.

To further improve the bonding between the conformal coating and all of the constituent parts and materials of the structure or assembly, the constituent parts and materials of the structure may be activated. Activation means that new chemical groups are formed on the surface of the material by surface tension, increasing the affinity of the surface for conformal coating. Typical gases used for plasma activation include O₂、N₂O、N₂、NH₃、H₂、CF₄、CH₄Ar, He or mixtures of the foregoing. As a result of performing activation and coating in the same chamber, significant savings can be further realized as compared to conventional conformal coating methods.

Finally, it is necessary to remove any trapped gas or water to achieve and maintain good adhesion between the conformal coating and all the components and materials in the complex three-dimensional structure or assembly. This allows the gas in the plasma process to penetrate to the core of the structure. This can be accomplished by baking the structure prior to placing the structure in the plasma chamber, as in conventional conformal coating techniques. The invention described herein enables outgassing to be performed at least in part in the same chamber as pre-cleaning, etching, and plasma polymerization.

The vacuum helps remove moisture from the structure, which improves adhesion and prevents problems encountered during thermal cycling during the life of the product. The pressure of the degassing may range from 10mTorr to 760Torr, the temperature may range from 5 to 200 ℃, and may be performed for between 1 and 120 minutes, but is typically a few minutes. Furthermore, by performing the pre-degassing and coating in the same chamber, significant cost savings can be realized over existing conformal coating schemes.

By appropriate selection of process parameters and gas mixing, cleaning, etching, and activation can all be performed in a single process step for certain combinations of materials and components.

Experiments have shown, for example, that conformal coatings can be used for electronic components, such as individual transistors or integrated circuits. Such individual parts may be coated after assembly into a large system part and may again be coated according to the method of the present invention. These coatings have also been found to be particularly suitable for both bare PCBs and assembled PCBs.

The conformal nanocoating of the present invention is therefore particularly advantageous in coatings of complex structures, where the complex structures may comprise 3D structures and/or combinations of different materials and/or components.

The method of the present invention allows different materials to be combined in a single nanocoating in the same process (time). The method of the present invention also allows for the application of nanocoating to more complex 3D structures.

In a preferred embodiment of the invention, the nanocoating is applied to a printed circuit board having components attached thereto, thereby providing a conformal coating of the assembly. In another preferred form, the complex sub-structures may be first coated with a conformal nanocoating and then interconnected to form a single complex assembly, which may have a subsequent nanocoating applied thereto to provide an integral conformal coating. The nanocoating as described in the present invention provides water, oil, salt, acid and flame protection on all surfaces and parts of a structure or assembly.

Experiments have shown that the nanocoating is also resistant to high temperatures in excess of 200 ℃.

The nanocoating also exhibits elasticity, which makes it suitable for flexible structures or applications requiring shock resistance.

The nanocoating described in the present invention also has important properties that are weldable by employing standard welding processes.

In another aspect, the invention also relates to the nano-coating of electronic and microelectronic components, integrated circuits, Printed Circuit Boards (PCBs), including dice and assemblies, using the above-described methods.

The invention also relates to the use of the above method to apply a nanocoating to all surfaces and parts of a structure, whereby the nanocoating is water, oil, salt, acid and flame resistant.

The invention also relates to the application of an elastically weldable nanocoating using the above method.

In yet another aspect, the invention relates to applying conformal nanocoating to three-dimensional structures of conductive and non-conductive portions and/or components of different materials. The coating has a thickness between 5 and 500nm, preferably between 25 and 250 nm. The conformal nanocoating is applied by the method described previously.

In yet another aspect, the present invention relates to a printed circuit board assembly having a conformal nanocoating as described above. The conformal nanocoating is applied by a low pressure plasma process.

Further advantages of the present invention will become apparent by reference to the following detailed description of exemplary embodiments, taken in conjunction with fig. 1 and 2, which illustrate one or more non-limiting aspects of the embodiments.

Drawings

In the detailed description, reference will be made to the accompanying drawings, which have the following contents:

FIG. 1 is a schematic illustration of a single electrode according to the present invention;

fig. 2 shows one embodiment of a multiple electrode arrangement according to the invention that may be suitable for a vacuum chamber.

Detailed Description

Example 1: electrode arrangement in a reaction chamber

Figures 1 and 2 show a preferred arrangement. The electrode arrangement for generating low-pressure plasma comprises a set of floating electrodes (1) which are hollow, curved and circular in shape and a vacuum chamber (5), the vacuum chamber (5) serving as a main body. The electrode (1) is provided with a liquid which can be cooled or heated for plasma treatment in a temperature range of 5 to 200 ℃, and preferably controlled at a temperature between 20 and 90 ℃.

A typical electrode (1) in this setup has a diameter between 5 and 50mm, a wall thickness of 0.25 to 2.5mm and is bent towards the end with a 180 ° turning circle, and the distance between the tubes before and after bending is between 1 and 10 times, preferably 5 times the tube diameter.

Power is applied to the electrode (1) through a connecting plate (2) mounted on a clutch plate (4). A thin insulating layer or protection (3) is applied between the clutch plate (4) and the chamber (5). The thickness of this layer is typically a few millimeters so that no plasma can be generated therebetween.

For example, by using a perforated metal container or tray (6) that can be pushed between the electrodes, a three-dimensional structure or device to which the nanocoating is to be applied is provided between the electrodes. A minimum distance of a few millimeters is preferably maintained between the electrodes and the substrate. The floating electrode in the above-described apparatus enables the application of a uniform three-dimensional coating in a single process step. It is not necessary to coat the top and bottom of the structure in separate steps.

The electrodes generate a high frequency electric field at a frequency of between 20kHz and 2.45GHz, typically 40kHz or 13.56MHz, preferably 13.56 MHz.

Such an electrode arrangement is provided in a CD1000 plasma system.

Example 2: low pressure plasma polymerized C3F6 embedded in circuit board for telephone

The assembled circuit board for the mobile phone was placed in the CD1000 plasma chamber for more than two minutes as described in example 1 and degassed at a pressure between 100 and 1000 mTorr. The plate was then cleaned and etched with Ar, and plasma polymerization was performed with C3F6 monomer at 50mTorr and room temperature for 10 minutes. The fluoropolymer conformal coating applied by this process measured to have a thickness of about 80 nm.

The circuit board is then exposed to several aging processes involving prolonged exposure to moisture, high temperatures, and salt fog (salt haze). It can be seen visually that the circuit boards with conformal nanocoatings showed significantly lower corrosion than the circuit boards without treatment. In performing electrical testing, it was also found that circuit board assemblies having nano-conformal coatings actually exhibited no electrical failures, which was significantly lower than circuit board assemblies without coatings.

Claims (44)

1. A method of depositing a conformal nanocoating on a three-dimensional structure or assembly of conductive and non-conductive elements, characterized in that the coating is deposited by a low pressure plasma process.

2. The method of claim 1, wherein the coating is deposited by a low pressure plasma polymerization process.

3. The method of claim 2, wherein a plasma cleaning and/or etching step is performed prior to the plasma polymerization process.

4. The method of claim 2, wherein the plasma polymerization process is preceded by a plasma activation step.

5. The method of claims 2-4, wherein the degassing of the structure or component is performed prior to the plasma polymerization process.

6. The method according to any one of claims 1 to 5, comprising the steps of:

a. the degassing according to claim 5, wherein the gas-permeable membrane,

b. plasma cleaning and/or etching according to claim 3, and

c. coating according to claim 1 or 2.

7. The method according to any one of claims 1 to 5, comprising the steps of:

a. the degassing according to claim 5, wherein the gas-permeable membrane,

b. plasma cleaning and/or etching according to claim 3,

c. activation according to claim 4, and

d. coating according to claim 1 or 2.

8. The method according to claim 7, wherein the cleaning and/or the etching and the activation are combined in a single process step.

9. The method according to claim 7 or 8, wherein the degassing, the cleaning and/or the etching and the activating are combined in a single process step.

10. The method of any one of claims 3 to 9, wherein all steps are performed in the same plasma chamber.

11. The method according to any one of claims 1 to 10, wherein the thickness of the coating is between 5nm and 500nm, preferably between 25nm and 250 nm.

12. The method according to any one of claims 1 to 10, wherein the plasma process is performed at a pressure of from 10 to 1000 mTorr.

13. The method according to any one of claims 1 to 10, wherein the plasma process is carried out at a temperature between 5 ℃ and 200 ℃, preferably between 20 ℃ and 90 ℃.

14. The method according to any one of claims 1 to 10, wherein the plasma process is performed at a frequency of 20kHz to 2.45GHz, preferably at a frequency of 40kHz, more preferably at a frequency of 13.56 MHz.

15. The method, as recited in any of claims 1-14, wherein RF power is continuously maintained during the plasma polymerization process.

16. The method according to any one of claims 1 to 14, wherein the RF power is pulsed during the plasma polymerization process, and the frequency of the pulses is typically between 1Hz and 100kHz, and the duty cycle is typically between 0.05 and 50%.

17. The method of claim 3, wherein the cleaning and/or the etching employ gaseous O₂、N₂、H₂、CF₄Ar, He or mixtures thereof.

18. The method according to claim 4, wherein the activation employs a chemical such as O₂、N₂O、N₂、NH₃、H₂、CF₄、CH₄Ar, He or a mixture thereof.

19. A method according to any one of claims 2 to 18, wherein gaseous polymerisable monomers or mixtures thereof mixed with the gases listed in claim 17 or 18 are employed.

20. The method of claim 19, wherein the monomer is produced from a gaseous precursor obtained by heating a liquid precursor or a solid precursor or a combination of the foregoing.

21. The method according to claim 19 or 20, wherein the monomers comprise halogen, sulfur, phosphorus, nitrogen and/or silicone.

22. The method of claim 19 or 20, wherein the monomer is derived from a precursor CF₄、C₂F₆、C₃F₆、C₃F₈、C₄F₈、C₅F₁₂、C₆F₁₄And/or any other saturated or unsaturated hydrofluorocarbon(s) (C)_xF_y）。

23. A method according to claim 19 or 20, wherein the monomer is derived from one or more precursors of trimethyl phosphate, triethyl phosphate, tripropyl phosphate or other phosphoric acid derivatives.

24. The method of claim 19 or 20, wherein the monomer is derived from one or more of the precursors ethylamine, triethylamine, allylamine, or acrylonitrile.

25. The method of claim 19 or 20, wherein the monomer is derived from an acrylate, methacrylate, or mixture thereof.

26. The method of claim 19 or 20, wherein the monomer is derived from a siloxane, silane, silazane, or mixtures thereof.

27. A method according to any one of claims 1 to 26, wherein the electrically conductive portion of the structure or component comprises a metal, such as copper, aluminium, silver or gold.

28. A method according to any one of claims 1 to 26, wherein the conductive portion of the structure or component comprises a semiconducting material or a conductive polymer.

29. The method according to any one of claims 1 to 26, wherein the non-conductive part of the structure or assembly comprises paper or plastic with or without glass fibre reinforcement, such as polyimide, polytetrafluoroethylene, silicone or polyamide.

30. A method according to any one of claims 1 to 26, wherein the non-conductive portion of the structure or component comprises a ceramic material such as glass.

31. The method according to any one of claims 1 to 26, wherein the three-dimensional structure or assembly is rigid.

32. The method according to any one of claims 1 to 26, wherein the three-dimensional structure or assembly is flexible.

33. Use of the method according to any one of claims 1 to 32 for coating electronic components and microelectronic devices.

34. Use of a method according to any of claims 1 to 32 for coating integrated circuits.

35. Use of a method according to any one of claims 1 to 32 for coating bare printed circuit boards.

36. Use of a method according to any one of claims 1 to 32 for coating printed circuit boards populated with electronic components.

37. Use of a method according to any one of claims 1 to 36 for depositing a nanocoating to provide water, oil, salt, acid and flame protection to all surfaces and parts of the structure or component.

38. Use of a method according to any one of claims 1 to 36 for depositing an elastomeric nanocoating.

39. Use of a method according to any one of claims 1 to 36 for depositing a nanocoating capable of being soldered therethrough.

40. A conformal nanocoating applied to three-dimensional structures of conductive and non-conductive portions and/or different material components.

41. The conformal nanocoating of claim 40, wherein the thickness of the coating is between 5 and 500nm, preferably between 25 and 250 nm.

42. The conformal nanocoating of claim 40 or 41, deposited using the method of any one of claims 1 to 32.

43. A printed circuit board assembly comprising the conformal nanocoating of any one of claims 40-42.

44. The printed circuit board assembly of claim 43, wherein the conformal nanocoating is deposited by a low pressure plasma process.